I need a really dramatic, beautiful instrumental song, possibly with a big crescendo.

The Adagio of Spartacus and Phrygia by Khachaturian is possibly one of the sexiest pieces of music ever written. It might work, but it could be a little to pensive for your needs. Give it a listen though -- it's beautiful, and yes - it has one huge crescendo at its climax.

Dramatic, so beautiful that you can't get it out of your head, but instrumental. 2 out of 3, she may want to consider arias. Generally always dramatic, moving, and with a melody that will stick in one's head easily. Many deal with love. Everything I can think of is about tragic love, but one thing I can think of is Rusalka's "Song to the Moon," by Dvorak, listen allt eh way to the end (I prefer the Renee Fleming version, she records well, even though I'm not a real fan of hers otherwise). There's also of course the Lakme Flower duet, Puccini and Verdi overtures. Instrumental I'd suggest Elgar's cello concerto performed by Jacqueline DuPre. Incredibly moving.

"A Saucerful of Secrets" by Pink Floyd. Use the version off the Ummagumma live disc. Start it about 7 minutes, 13 seconds into the song. This is the part of the song called "Celestial Voices," and it may be labeled as such on your CD. Total running time is about 5 minutes, 30 seconds. Dramatic, beautiful, and sweeping, all at the same time. I've considered getting married just so I can have this piece of music played at my wedding.

http://en.wikipedia.org/wiki/Classical_Gas. Performed Live.

Wagner's "Prelude and Liebestod" from Tristan und Isolde is highly dramatic and lovely, IMHO. Some portion of that could work for her.

wow, i was going to say "glosoli" as well. the first time i heard that song i almost drove off the road when the end part hit.

I can't recall specific tracks, but off Sufjan Steven's cd Illinoise, there are some incredibly beautiful songs, both with and without lyrics. Yeah, you should totally use John Wayne Gacy, Jr.! (Note: Actually you shouldn't, nor should you use anything else from that album, good though it may be.)

We used Sigur Ros' "Glosoli", from their latest album "Takk". It worked out really well. It was also a personal song for us, though, so YMMV.

Heh. Banjo and I used a string quartet cover of a Nirvana song (All Apologies) for our walk. In lieu of a crescendo, we had people slowly realizing what song we were playing and then slowly getting the joke from the song's chorus.